Surface and deep ocean circulation through time- LGM Flashcards
Evidence for ice activity-indicating ice movement
- Erratic boulders
- Striae
What controls long-term climate chage?
Variation in Earth’s orbital parameters- Milankovitvh theory
Eccentricity
Variation of the Earth’s path around the sun- 100ka and 400ka
Obliquity
Variation of Earth’s rotational axis with respect to its rotational plane around the sun- 41ka
Precession
Wobble of the rotational axis - 19-24 ka
1-6% eccentricity (circular to elliptical)
Takes 100,000 years
Aphelion
Distant path position to the sun
Perihelion
Close path position to the sun
Earth’s axial tilt varies from 24.5 to 22.1 degrees
Every 41,000 years
What impact does axial tilt (obliquity) have?
Effects the distribution of solar radiation on Earth’s surface- when tilt decreases, polar regions receive less sunlight
Impact of changing precession?
Alters which hemisphere receives more sunlight during the summer
Evidence for the Milankovitch cycles?
Assemblages of particles (biogenic or lithogenic) within the ocean- sedimentation
Most frequently used ‘tool’ used to reconstruct ocean circulation patterns
Planktonic and benthic foraminifera
Oxygen isotopes incorporated in foraminifera that reflect T an S of sea water
18O and 16O incorporated as part of the CaCO3 test
Two carbon isotopes incorporated into the test of foraminifera
13C and 12C
The older the water mass…
The higher the 12C concentration in the water and the foraminifera tests
Which C isotope end up in organic tissue in a higher amount?
12C
When organisms die…
Organic matter sinks to the floor taking 12C rich tissue with it- extracting 12C from the surface ocean and oxidizing organic matter on its way down the water column and releasing 12C back into the ocean at depth= BIOLOGICAL PUMPING
NADW- isotope values
NADW starts with high carbon isotope values- on its way towards the Pacific–> addition of 12C by oxidizing organic matter DECREASES CARBON ISOTOPE VALUES
Carbon isotope signature of AABW
Very low carbon isotope values
Oxygen isotope- evaporation
During evaporation 18O and 16O enter the vapor phase at a different rate. 16O more enriched in water vapor that 18O
16O evaporation
Evaporates much quicker than 18O
Oxygen isotopes in foraminifera are controlled by 3 parameters:
- Temperature
- Variation in size of continental ice masses
- oxygen isotopes of the ambient seawater (out of which the foraminifera calcifies its test)
Difference in oxygen isotope values between surface and deep ocean:
Reflect the temperature gradient between surface and deep ocean
Change in oxygen values in foraminiera with temp?
Warmer temps= favour 16O incorporated into CaCO3 test
Colder temps= favour 18O
Consequences of waxing and waning of continental ice masses for the ocean:
- A lot of 16O extracted from ocean and stored in the ice leading to higher oxygen isotopes in water
- Extracted water led to lowered sea level
Piston Corer
Can retrieve sediment cores from the soft seabed to depths of 30m. The piston prevents water from entering the core barrel- preserving sediment layers
Gravity Corer
Retrieves sediment cores from soft- medium firm sediments from a few m to tens of m. Lowered and collected sample by its weight - cheaper than piston corer but shorter samples
Box corer
Can collect LARGE UNDISTURBED sediment samples however only on the top layer of sea floor. Collection by weighted box and two hinged spade doors
Multi corer
Multi corer for short sediment sequences. Provides excellent surfaces sediment/bottom water samples. Better than box corer, but more fragile, entailing risk of destruction. Length of sediment section depends on design (50-60cm max)
Steps leading from LGM–> Holocene
- Deglacial step- terminated LGM
- B-A Oscillation
- Younger Dryas (YD)
B-A Oscillation
Bolling-Allerod Oscillation; two short warm pulses separated by a short cold event
IRDs during the glaciation
Ice rafted debris (IRD) - icebergs drifted off to Portugal leaving a trail of IRD- implies meltwater being around at the sea surface
Sediment transport associated with Heinrich events
IRD released from icebergs and suspended sediment in meltwater
Surface ocean conditions-LGM
Much colder N.Atlantic with a few icebergs around an expanded sea ice cover
–> Gulf Stream= weakened (most of the flow re-routed eastward)
Meltwater world (Heinrich Event 1)
Cold in general, additional cooling in some locations, flotillas of icebergs around accompanied by meltwater
The Holocene
Modern warm period
Planktonic foraminifera
Reflect surface ocean conditions
Benthic foraminifera
Reflect bottom ocean conditions
Projection plane
Individual locations projected onto the projection plane according to location and water depth
Sediment dating
Used to establish the rate a particular section has been deposited
Time slice approach
Defining sediment sections that contain info from a specific time period
NADW- LGM
The likely sources areas for glacial NADW shifted to an area south of Iceland
Meltwater period (Heinrich Event 1) - NADW
Meltwater from ice caps melting reaches source areas of NADW reducing salinity- NADW formation REDUCES.
Changes in T/S properties in the N-Atlantic
Large volume of ice= water volume reduced and salinity increased (1psu) - affects convention in areas of Deepwater formation
Younger Dryas (YD)
Return to glacial conditions in less than 100 years
–> Maybe caused by a reduction in North Atlantic Conveyer Belt
